You are not along. Two other forum posts have spun up today with this issue. Also there is a forum post from 2 years ago. Only remedy to get you working ASAP is to recreate credentials, and then in all integrations that use credentials switch to the new. Futhermore, this looks to be an issue that happened exact same day 2 years ago. Feels like n8n has something wrong with certificates and Microsoft OAuth2 handshake. It looks like they can fix on the back end and then restart your instance but you may get yourself working faster by recreating.
Two Additional forum posts from today:
Error When Connecting Microsoft Outlook OAuth2 API - Questions - n8n Community
Microsoft Outlook Client authentication failed - Questions - n8n Community
Forum Post from Two Years ago:
Issue with MS Oauth2 on n8n cloud - Questions - n8n Community